The outcome could see the Black Sea state move closer to Russia. Bulgarians go to the polls in a presidential election run-off on Sunday – and a man friendly towards the Kremlin is expected to win.
Former air force commander General Rumen Radev, who stands as an independent allied to the Socialist Party, has expressed tolerance for Russian moves in Ukraine.
He also favours easing Western sanctions on Moscow.
Bulgaria already relies on Russia for most of its energy, military equipment and tourism revenues.
Two opinion polls show ruling party candidate Tsetska Tsacheva is trailing Rudev.
A win for the independent would put Bulgaria at odds with its EU and NATO allies.
It would also likely trigger months of political instability, with Prime Minister Boiko Borisov, of the centre-right GERB party, having said that he’ll step down if his candidate loses.
Rudev is winning over Bulgarians frustrated with a political elite they see as corrupt and selfish.
Almost 10 years of the country joined the EU; many of its people are also feel their problems are too low down the priority list in Brussels.
In Bulgaria, the government and parliament wield most power but the president can veto legislation once, appoint some key officials and is also chief commander of the armed forces.